This application claims benefit of Japanese Patent Application No. 2000-163824 filed on May 31, 2000, the contents of which are incorporated by the reference.
The present invention relates to portable terminals such as portable telephone sets and to a communicative base station switching system of a portable terminal for effectively switching the frequency thereof over to the communicative frequencies of neighbor base stations in order to switch the communicative base station.
Japanese Patent Laid-Open No. 9-284826 discloses a portable terminal, which switches the prevailing communicative base station over to a next frequency base station while in motion.
FIG. 10 is a view showing the relationship of the communicative areas covered by a portable terminal and base stations in the prior art. As shown in the Figure, a plurality of base stations A, B, C . . . are located around a communicative base station O. The communicative base station O and the neighbor base stations A, B, C . . . have fixed communicative areas with borderline parts thereof overlapping one another.
A portable terminal 10 such as a portable telephone set is located in the area of the communicative base station O and communicating with the base station O.
The communication of the portable terminal 10 with the communicative base station O and the neighbor base stations A, B, C, . . . is made at different frequencies.
When the portable terminal 10 in motion and in communication with the communicative base station O gets out the area of the communicative base station and enters one of the neighbor base stations A, B, C, . . . , it switches the base station 10 over to the maximum received power level neighbor base station after receiving the received power measurement of the neighbor base stations.
At the time of the switching, the portable terminal 10 measures the received power levels by switching frequencies quickly with a synthesizer at a predetermined frequency. The received power level measurement requires a predetermined time.
However, since the portable terminal 10 switches frequencies quickly, a problem is posed that the characteristics of some synthesizer are subject to deterioration.
In addition, in the portable terminal 10 the switching to the neighbor base stations always requires a predetermined time, and a problem is thus posed that switching process software is subject to great burden.